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Thermochemical structures beneath Africa and the Pacific Ocean


Large low-velocity seismic anomalies have been detected in the Earth's lower mantle beneath Africa and the Pacific Ocean that are not easily explained by temperature variations alone1,2,3,4,5,6,7,8,9,10,11. The African anomaly has been interpreted to be a northwest–southeast-trending structure3,4,5,7 with a sharp-edged linear, ridge-like morphology9,10. The Pacific anomaly, on the other hand, appears to be more rounded in shape1,2,3,4,6,7,11. Mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes12,13,14,15,16,17,18,19. It has not been shown, however, that such models can lead to thermochemical structures that satisfy the geometrical constraints, as inferred from seismological observations. Here we present numerical models of thermochemical convection in a three-dimensional spherical geometry using plate velocities inferred for the past 119 million years20. We show that Earth's subduction history can lead to thermochemical structures similar in shape to the observed large, lower-mantle velocity anomalies. We find that subduction history tends to focus dense material into a ridge-like pile beneath Africa and a relatively more-rounded pile under the Pacific Ocean, consistent with seismic observations.

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Figure 1: Perspective views of two tomography models 4,7 and geodynamic compositional fields at the end of the calculations, corresponding to the present day.
Figure 2: Temperature and tomography maps.
Figure 3: Effect of initial condition.


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We thank C. Lithgow-Bertelloni and T. Becker for furnishing us with the plate velocity data used in these calculations. We also thank J. Ritsema and E. Garnero for discussions and for help in generating figures for this manuscript. This work was supported by the David and Lucile Packard Foundation and the National Science Foundation.

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Correspondence to Allen K. McNamara.

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McNamara, A., Zhong, S. Thermochemical structures beneath Africa and the Pacific Ocean. Nature 437, 1136–1139 (2005).

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